Recently, electric apparatuses such as an automotive product and an electric tool have been downsized and light-weighted. This market trend entails a power source of these electric apparatuses, i.e., a motor mounted to these apparatuses, to be also downsized and light-weighted.
A dc motor having brushes and a commutator is often employed to these apparatuses. The dc motor comprises the following elements in general:                a stator producing magnetic field; and        a rotor facing the stator via annular space.As the rotor, an armature including a commutator and an iron core wound with coils is employed. The armature should be powered in order to drive the dc motor. For that purpose, brushes—connected to an outer power source with lead wires—are brought into contact with the commutator.        
The Japanese Patent Application Non-Examined Publication No. H11-341723 discloses one of conventional motors of this kind. FIG. 8 shows the motor disclosed, which comprises four magnetic poles, five teeth of an armature, ten segments of a commutator. The publication also discloses a method of winding coils as follows, which is illustrated in FIG. 9. The coils extended from segments of the same phase are wound on slots (teeth) of the same phase, thereby preventing the output of a motor from decreasing.
However, the structure discussed above shorts segments S2 and S3, or S5 and S6 with brush B1 or B2 depending on a position where the brushes contact with the segment. As a result, current does not run through some coils, which are marked with a circle in FIG. 8. In other words, numbers of inactive conductors are produced, thereby reducing the output of the motor, which has been an obstacle to the progress of downsizing and weight reduction of a motor.